U–Pb evidence for polymetamorphic history of Huronian rocks within the Grenville front tectonic zone east of Sudbury, Ontario, Canada

Abstract

This paper presents U–Pb geochronological data for zircon, titanite, monazite, apatite, allanite, and rutile from Street Township east of Sudbury that help unravel two main facets of the geology of the Grenville Front Tectonic Zone (GFTZ). They document the presence of extensive Huronian Supergroup magmatic (ca. 2480–2460 Ma) and sedimentary rocks inside the GFTZ, and highlight the significance of bimodal magmatism in the genesis of the Huronian Supergroup. The data also provide further insights into the complex orogenic evolution of the GFTZ, showing in particular that amphibolite facies metamorphism, migmatization and deformation during Grenvillian compression at 995–980 Ma overprinted, but did not totally erase, the memory of earlier metamorphic events at ca. 1720 and 1470 Ma. From the point of view of U–Pb systematics, the results emphasize the nearly ubiquitous state of isotopic disequilibrium in these rocks. Most of the minerals investigated comprise two or more generations and are generally in a state of partial resetting because of the superimposed effects of younger events. The preservation of pre-Grenvillian ages in titanite and allanite demonstrates that processes related to final Grenvillian collision and exhumation occurred at great speed. Only apatite and to some degree rutile yield ages that are clearly post-tectonic, thereby reflecting the final stages of exhumation and cooling of the region.